The head of Russia’s GRU military intelligence agency, General Igor Korobov, has died aged 62, Russia’s defence ministry says.
Gen Korobov, who took up the post in 2016, is said to have died after “a serious and long illness” on Wednesday.
The GRU was this year linked to a nerve agent attack in Britain on Russian ex-spy Sergei Skripal and his daughter.
Gen Korobov is understood to have faced criticism by Russian officials over the failure of the operation.
The attack on Mr Skripal, 66, and his 33-year-old daughter, Yulia, in Salisbury on 4 March led to them requiring weeks of hospital treatment.
The poisoning is alleged to have been carried out by suspects Alexander Mishkin and Anatoly Chepiga, both identified by British authorities as GRU operatives, and “almost certainly” approved by the Russian state.
Russia denies the allegations. The UK and its Western allies expelled more than 150 Russian diplomats in retaliation for the attack.
Vice-Admiral Igor Kostyukov, who has commanded Russian forces in Syria, will serve as interim GRU chief. He had already taken over while Gen Korobov was ill, military sources told Russian media.
The appointment of a permanent chief is a matter for President Vladimir Putin.
The GRU, or Main Intelligence Directorate, is the intelligence arm of the Russian military, tasked with carrying out undercover international operations.
Gen Korobov had received the Hero of Russia medal – the state’s highest award.
In December 2016 the US added Gen Korobov to its list of senior Russian officials subject to sanctions, accusing him of involvement in computer hacking.
Other Western sanctions target Russians accused of helping the separatist insurgents in Ukraine.
Russia has two other main spy organisations: the Federal Security Service (FSB), mainly involved in internal security, and the Foreign Intelligence Service (SVR), whose role is similar to that of Britain’s MI6.